An army of citizen scientists has helped the professionals understand how a tiny 'alien' moth is attacking the UK's conker (horse-chestnut) trees, and showed that naturally-occurring pest controlling wasps are not able to restrict the moth's impact.

The study's conclusions are published this week in the open access scientific journal PLOS ONE.

No bigger than a grain of rice, the horse-chestnut leaf-mining moth has spread rapidly through England and Wales since its arrival in London in 2002. The caterpillars of the moth 'tunnel' through the leaves of conker trees, causing them to turn brown and autumnal in appearance, even in the height of summer.

In 2010 thousands of 'citizen scientists' were asked by two professional ecologists to collect records of leaf damage from across the country as part of a project called 'Conker Tree Science'.

The results show that over the last decade the moth has spread from London to reach almost all of England and Wales. Investigating the data further the scientific team concluded that it takes just three years from the first sighting of the moth in a particular location to maximum levels of damage to the horse-chestnut trees being recorded.

In a follow-up experiment, many of the citizen scientists, including hundreds of school children, followed instructions to rear the moth by sealing the infested leaves in plastic bags and waiting for the insects to emerge. The results reveal that the tiny pest controllers ('parasitiod' wasps) that prey upon the caterpillars are not present in high enough numbers to control the moths.

This is an image of horse-chestnut leaf-mining moth.

(Photo Credit: Dave Kilbey)

Dr Michael Pocock, an ecologist at the Centre for Ecology & Hydrology and lead author of the research paper said, "This is the sort of science that anyone can do. By taking part the public are doing real science – and the publication of this scientific paper is a demonstration of how seriously citizen science is now taken by the community of professional scientists."

Co-author Dr Darren Evans, a conservation biologist at the University of Hull said, "This work could have been done by paying research assistants to travel the country and collect records, but by inviting thousands of people to get involved we, together, were able to pull this off much more cost-effectively."

Dr Pocock added, "It seems almost like magic for children and other people to put a damaged leaf in a plastic bag, wait two weeks and then see insects – the adult moths or their pest controllers – emerge, but making these discoveries was a valuable contribution to understanding why some animals become so invasive."

Dr Evans added, "We have been challenged by other professional scientists as to whether 'ordinary people' can make accurate observations, suitable for real science. Of course they can – and we tested this in our study. So thank you to the thousands of participants because together we were able to do this science."

Unlike some other citizen science projects that use biological records submitted by members of the public for long-term monitoring, the Conker Tree Science project set out to test two specific hypotheses over the course of a year. The authors suggest that this approach can be developed to examine a range of environmental problems.